sequence problems and partial sums
$s_3=a_1+a_2+a_3\le a_1+a_2+a_2=a_1+2a_2$ (using the fact that $a_3\le a_2$ for the inequality step).
You can do an analogous thing for the other inequality.
$s_3=a_1+a_2+a_3\le a_1+a_2+a_2=a_1+2a_2$ (using the fact that $a_3\le a_2$ for the inequality step).
You can do an analogous thing for the other inequality.